In a general wireless communications system, even when the bandwidth for each of an uplink and a downlink is set up differently, only a single carrier is being taken into consideration. For example, based on the single (or unique) carrier structure, the number of carriers configuring each of the uplink and the downlink respectively corresponds to 1. And, accordingly, a wireless communications system, wherein the bandwidth of the uplink and the bandwidth of the downlink are generally symmetrical to one another, may be provided.
The ITU (International Telecommunication Union) is requesting the candidate technology of IMT-Advanced to support a more expanded bandwidth as compared to that of the conventional wireless communications system. However, with the exception for a select area throughout the world, there still exist many difficulties in allocating a large bandwidth frequency. Therefore, in order to resolve such difficulties, a carrier aggregation (also referred to as bandwidth aggregation or spectrum aggregation) technology is currently under development. The carrier aggregation technology corresponds to a technology developed for an efficient usage of small segmented bands. More specifically, a plurality of small bands is physically grouped in the frequency domain so that the group of segmented bands can be logically used as one large band.
The carrier aggregation technology is adopted so as to support an increasing throughput, to prevent an increase in the manufacturing cost caused by the addition of a broadband RF device, and to ensure backward compatibility with the conventional system. More specifically, carrier aggregation refers to a technology enabling a user equipment (or terminal) and a base station to exchange data to and from one another through multiple groups of carriers of a bandwidth unit defined by the conventional wireless communications system (e.g., the 3GPP LTE release 8 or 9 system in case of the 3GPP LTE-Advanced system). Herein, the carrier of the bandwidth unit defined by the conventional wireless communications system may be referred to as a component carrier (CC). For each of the uplink and downlink, a carrier aggregation technology using at least one or more component carriers (CCs) may be applied. The carrier aggregation technology may include a technology supporting a system bandwidth of up to a maximum of 100 MHz by grouping a maximum of 5 component carriers, even when one component carrier supports the bandwidth of 5 MHz, 10 MHz or 20 MHz.